As one of the fields to which the present invention is applicable, a liquid crystal display (LCD) is a device that generally displays an image by injecting a liquid crystal material between an upper substrate and a lower substrate, changing orientations of liquid crystal molecules by applying different electric potentials to pixel electrodes and common electrodes to form an electric field, and adjusting the transmissivity of light according to the orientations of the liquid crystal molecules, wherein the upper substrate has common electrodes, color filters and the like formed therein, and the lower substrate has thin film transistors, pixel electrodes and the like formed therein.
Since a liquid crystal display panel is a passive element that does not voluntarily emit light, a backlight unit is necessarily installed in the liquid crystal display panel to supply light. In general, the backlight unit includes a light source supplying light; a diffuser plate or a light guide plate converting a line light source or a point light source into a surface light source; and a variety of optical films used to improve optical performances.
The optical films used in the backlight unit includes a collimating film used to improve brightness, a diffusing film having an effect to shield defects of the backlight rear or bright lines of the light source, etc.
Meanwhile, the collimating film has a lens structure arranged periodically to deflect a light path at one surface thereof. A generally used lens structure includes a prismatic lens, a semi-cylindrical lenticular lens, micro lens array, a Fresnel lens, etc.
These lens structures have functions to collimate light emitted from a light source towards the front of a display to improve brightness. However, the collimating films having these lens structures have problems regarding the Moire phenomenon emerging from the periodicity, the wet-out phenomenon caused by the lack of gap, the Newton ring phenomenon of contour patterns due to the changes in air gap between two adjacent films, and also has a disadvantage in that surface defects occurs on a screen due to the above-mentioned phenomena.
Therefore, in order to solve the above problems, there have been attempts to relieve the regularity of a lens structure such as prism or lenticular lens. As one representative example, there has been proposed a method for relieving the regularity of a lens structure by forming a shape of a lens structure in a mold using bite processing, throwing beads having a size of several micrometers to several tens micrometers to the mold, and sanding the lens structures to form a random secondary structure on the lens structure formed in the mold.
However, this method has problems in that it is difficult to expect a position where a secondary structure is formed due to the difficulty in controlling a position where beads are thrown, and optical performances are deteriorated since beads are hardly thrown to a concave surface of the mold due to the air turbulence, but sanded mainly on a convex surface of the mold. Also, the reliability of products is degraded since the reproducibility is not maintained at every sanding process. Furthermore, optical films prepared in these methods show somewhat improved Moire or Newton ring phenomena, but have problems in that a collimating effect of the lens structures is deteriorated and the haze is increased due to the random formation of secondary structures.
Meanwhile, backlight units including a film-laminate which is prepared by stacking a plurality of films such as a collimating film, a diffusing film, a protective film and etc., have been recently used. These backlight units have problems in that surface defects are induced as the stacked films adhere (block) to each other.